Before everyone starts buying crimpers I would see how you go just using needle nose pliers. I have a crimper but I often just grab my pliers because they're closer and I can do the job just as good provided it's just thin wire (not too thin, not too thick).
..and you don't care too much about long-term reliability.
Crimping is all about achieving an airtight compression of wire and contact. Using a totally wrong tool is unlikely to achieve this
This is true, but for home projects not such a big issue provided the terminals are accessible for maintenance. As posted previously good crimpers cost quite a bit of money, so for the hobbyist going for the needle nose pliers is worth a try imo.
It depends on how one values their time. I have done my share of plier or shitty stripper/lug crimper/wire cutter crimped 1/4" spades, and about 80% of them pull off eventually.
For insulated crimp on spade/ring terminals, the TE ratcheting crimpers (especially with PIDG series lugs) are great.
http://www.alliedelec.com/search/productdetail.aspx?sku=70089831&mkwid=oPlqAWeX&pcrid=48651000259&gclid=CJHLyN_n28ACFScV7Aod4QMATAThey arent $500, but for $140, I crimp a terminal onto the stripped wire, which takes all of 5 seconds, instead of taking a pair of plies to a terminal, spending ~20 seconds fiddling with it until I'm satisfied its "adequate" then when I try to take the terminal off, I have to carefully make sure I tug only at the terminal and put 0 force on the wire, because if I do, its going to just yank the wire out, and then I have to go dig the pliers back out to try and separate the terminals with next to nothing to hold onto now, etc...
With a lug meant for 18 awg wire, and some cheap and nasty zipcord type 18 awg wire, generally the wire breaks before it slides out of the terminal.